1. Field of the Invention
The invention relates to a method of measuring the thickness of test articles by pulse reflection ultrasonics, the transit time as determined by the signal triggering the ultrasonic transmission and by an ultrasonic signal reflected at the end of the measurement path length, being evaluated as a measure of the distance travelled by the ultrasonics. The invention also relates to an apparatus for the performance of the method.
2. Description of the Related Art
The underlying principle of pulse reflection ultrasonic measurement of material thickness is as follows:
A test head coupled with the material surface either directly or via a buffer zone introduces an ultrasonic pulse into the material. The pulses reflected on the surfaces of the material are received by the test head at intervals of time which correspond to the distances travelled by the sound. These time intervals are measured as an indication of the distances travelled in them by the sound, such distance being calculable by reference to the speed of sound. The time count is made between the transmitted pulse and the rear wall echo pulse in the case of a direct introduction of the sound, whereas when a buffer zone is used, the surface echo is included in the measurement. To ensure that the voltage of the ultrasonic pulses to be evaluated is far enough away from background noise, in the conventional known measuring procedures, as are used in the thickness monitor 1214-1 produced by the firm Karl Deutsch Pruf und Messgeratebau Wuppertal, an adjustable fixed analog comparison threshold is set for each ultrasonic signal to be evaluated, and when such threshold is exceeded by the ultrasonic signal, the measuring operation starts and stops, the measured value of material thickness corresponding to the time elapsing between the starting and stopping of the measuring operation (see, for example, page 8, column 3 of the Prospectus P1140 of May, 1973, of the firm Karl Deutsch Pruf und Messgeratebau regarding ECHOGRAPH 1140).
The time measurements can be made in either analog or digital form. In analog measurements a voltage rising in proportion to time is delivered as a measurement value, the voltage subsequently being digitized. An example of apparatus which operates in such a manner is the thickness measuring apparatus sold under the description "General purpose thickness gage model 5222" by Panametrics, Inc. of Waltham, Mass. (USA). These methods, with the use of analog time measurement, offer the advantage of infinitely fine resolution, but their accuracy of measurement is limited by the instability of the analog components (temperature difference, offset and so on) and by the subsequent digitization of the measured value (limited number of counting steps), so that the precision obtainable with a single measurement is, at most, 0.1 mm. The measurement range which can be evaluated also limits measurement accuracy since a voltage range of, at most, 15 volts can be used as a time-proportional signal. Consequently, the conventional wall thickness measuring devices using analog time period measurement operate only up to a maximum measured path length of 100 mm.
For digital propagation time measurement, a digital propagation time counter is used having a resolution which depends upon the counting frequency selected. In most of the known measuring systems, frequencies of up to 30 MHz are used so that the resolution for a single measurement is also 0.1 mm.
There are no advantages in having a finer resolution since the accuracy of measurement depends to a large extent upon the amplitude and frequency of the ultrasonic signal, for the starting and stopping of the system depends upon when the rising edge of the evaluated ultrasonic oscillation exceeds the analog comparison threshold. Since pulse voltage amplitude depends not only upon coupling conditions but also upon the thickness of the material, it is impossible to guarantee that the phase of the ultrasonic oscillation is always the same when the fixed comparison threshold is exceeded; consequently, this reason also makes it impossible for measurement accuracy to be any higher than 0.1 mm for a single measurement, likewise with a digital travel time value as in the input of the above-mentioned thickness monitor 1214-1.
To reduce measurement inaccuracies caused by different phase relationships, measuring methods for static coupling were developed, with the measurement operation extending over a number of test cycles. In U.S. Pat. No. 4,388,830 to Narushima et al, such a method for measuring the thickness of test articles is disclosed wherein clock pulses are phase-shifted by a predetermined amount of 2.pi./N (where N is an integer not less than 2) upon completion of each measurement of the time period to improve accuracy. In these methods the analog or digital propagation time from a number of testing cycles is totalized, so that measurement accuracy is increased statistically in accordance with the number of testing cycles used. The aforementioned thickness measuring apparatus of Panametrics, Inc. is such an apparatus for static wall thickness measurement with analog time measurement. A method for enhancing accuracy by computational averaging computing is disclosed in U.S. Pat. No. 4,471,657 to Voris et al, in connection with a digital ultrasonic stress measuring apparatus. The disadvantage of these methods is that a large number of testing cycles are needed for a single measurement result, a factor which slows down the measuring operation. For dynamic thickness measurements in which the testing head and the test article move relative to one another at speeds of up to 3 meters/second, the varying coupling conditions and the fact that the pulse repetition rate is limited by the propagation time of sound, make it impossible to resort to measurements which extend over a number of cycles. By effecting the measurement of the elapsed time period with respect to a plurality of test cycles, measurement inaccuracies can be reduced, but not brought to zero.
It has already been suggested among experts that to compensate for phase error, the rising edge of the ultrasonic signal should start a first propagation time counter upon passing by the analog comparison threshold. The falling edge of the same half-wave starts a second propagation time counter upon passing the same comparison threshold. Similarly, the first counter is stopped by the rising edge of the next ultrasonic signal and the second counter is stopped by the descending edge of the latter signal. The two measured values of the counters are averaged and outputted as one measured value. It is alleged that with this method, if the digital propagation time counter has a counting frequency of more than 300 MHz, a measurement accuracy of more than 0.01 mm is possible. However, a major uncertainty of this method is that because of variations in the amplitudes of ultrasonic pulses consisting of more than a single oscillation, the fixed analog comparison threshold is not exceeded reliably by the first half-wave but may in some circumstances be exceeded by the higher second half-wave, with the result of a measurement error in the form of half the wavelength or a multiple thereof. Another disadvantage is the substantial outlay on counters, since two counters with subsequent averaging are needed for each wall thickness measured value.
A thickness-measuring method and apparatus are described in DE-OS-2853170 which can provide measurement accuracies of better than 0.01 mm in both static and dynamic testing despite fluctuations in ultrasonic signal amplitudes, that is, for heavily damped wide-band oscillators providing a single oscillation and for less attenuated narrow-band oscillators providing a number of oscillations, without measurement values being distorted by phase inaccuracies and phase jumps. In this method, the threshold values are derived automatically, in the form of a reference voltage lower by a preadjustable difference than the peak value of the first positive or negative half-wave of the two ultrasonic pulses between which it is required to make the time measurement, and the latter pulses are delayed and compared with the respective threshold derived from them, with the generation of control signals to set the measuring periods for the propagation time measurement.
Since in this known method only the pulses fitting integrally into the time period are counted, a counting frequency in the region of 300 MHz is necessary if the required accuracy of measurement is to be achieved. Circuit arrangements operating on frequencies as high as this are, because of their heavy current consumption, unsuitable for portable battery-operated devices.